1. Field of the Invention
This invention relates to an output circuit having a fail-safe function, and more particularly to an output circuit having an output current detecting circuit for detecting a current through a load for a feedback control and a fail-safe function which operates for safety upon occurrence of an electrical short-circuit between the load and the output current detecting circuit.
2. Description of the Prior Art
A conventional fan motor control device having an output current detecting circuit is shown in FIG. 3. An air conditioner switch signal and a water temperature sensor signal are input to the fan motor control unit. A linear solenoid driving signal is output from the fan motor control unit to a solenoid L. A relationship between the water temperature and the solenoid current or fan revolutions per minute is shown in FIG. 4. In this device, a solenoid current is detected by a resistor R. One end of the resistor R is connected to the solenoid L. The other end of the resistor R is connected to ground in the fan motor control unit. The solenoid current is equal to the current through the resistor R. Therefore, it is possible to determine the solenoid current from the voltage across the resistor R.
The solenoid L is controlled by a feedback of the voltage to obtain a constant revolution of the fan. The resistor R is one part of an output current detecting circuit.
If the line between the solenoid L and the resistor R is short circuited to ground, the voltage across the resistor R becomes zero. Accordingly, the fan motor control unit regards the solenoid current as being zero so that the fan motor unit raises the solenoid current up to the maximum value. In this case, the number of fan revolutions will be low so the motor vehicle engine will be overheated.